FIG. 1 illustrates a gas turbine engine. A digital electronic control, indicated by block 1, controls several actuators of the engine by means of signals delivered on lines 2A-D. These actuators include the following.
A first actuator 6A opens and closes a bleed door 9 which bleeds pressurized air 11 from booster stage 14. The booster 14 is a low-pressure compressor, and bleeding is required in order to match the output of the booster, at point 16, to the input requirements of the high-pressure compressor 18. Door 9 is commonly referred to as a variable by-pass valve (VBV).
A second actuator 6B operates variable stator vanes 26 (VSV's), which are shown in more detail in FIG. 2. Varying the angle A by rotation of the VSV's, indicated by circular arrows 22, allows one to control the direction of the airstream 24 which enters the compressor blades 27, thereby controlling the angle of attack of the compressor blades 27. VSV's are used to improve the performance of the compressor under acceleration.
A third actuator 6C in FIG. 1 controls a valve 30 which blows hot (or cold) air 33 upon turbine casing 36 in order to expand (or shrink) the casing 36 to thereby control the clearance 39 between turbine blades 41 and the casing 36. The air 33 is commonly bled from the high-pressure compressor 18. It is desirable to maintain as small a clearance 39 as possible in order to minimize leakage through the clearance. Leakage represents a loss because the leaking air imparts virtually no momentum to the turbine blades 41, and the energy in the leaking air is wasted.
A fourth actuator 6D in FIG. 1 controls a fuel valve 43 which controls the amount of fuel delivered to combustors 44.
In addition to these four types of actuator, other types are also in use in gas turbine aircraft engines. For example, there are actuators involved in the thrust reversing system, in exhaust nozzles which are variable in area, and in thrust vectoring systems used in vertical takeoff and landing (VTOL) aircraft.
As stated above, lines 2A-D deliver control signals to the actuators 6A-D, and the signals generally take the form of analog electrical signals, instead of digital signals of either the serial or parallel type. Consequently, a digital-to-analog conversion must take place at a location between the control 1 and the actuators 6A-D because the control 1 in FIG. 1 is of the digital type: the control contains a digital computer (not shown in FIG. 1), which processes and stores data in digital form. Digital-to-analog converters (D/A's) 60 in FIG. 3 accomplish this conversion, and are described later in more detail.
In addition to driving the actuators described above, the control 1 receives signals on lines 61 from sensors (not shown) on the engine which indicate engine operating conditions, such as temperatures, pressures, rotational speeds, and stator vane positions. The control uses these sensor signals to compute the signals to be sent to the actuators, and to compute other data for other purposes.
It is desirable, especially during ground-based testing of the engine, to monitor the signals produced by selected sensors, and, further, to perform the monitoring at a recording station 63 located remote from the engine. At present, two types of signal interface, indicated by line 65, are commonly used in this monitoring, and they carry digital signals between the control 1 and recording station 63. A first type uses the RS 232 protocol, and a second type uses the ARINC 429 protocol. Details concerning the ARINC 429 Protocol are found in Specification 429-9, dated September, 1985, and available from Aeronautical Radio Incorporated, located in Annapolis, Md. Details concerning the RS 232 Protocol are found in Electronic Industries Association Standard RS-232-C, dated August, 1969, and the Association is located in Washington, D.C. However, both protocols have limitations. For example, the ARINC protocol allows one to reach only about 100 random access memory (RAM) locations, and allows an update time (i.e., the shortest time allowed between consecutive readings of the data) in the range of 60 to 240 milliseconds (msec.)
In contrast, the RS232 protocol can read all RAM locations in the digital electronic control, but the update rate can be slower, about 125 to 250 msec. It is sometimes desirable to read all memory locations, and at a faster rate, than allowed by these two types of interface.
Further, with both protocols, the digital-to-analog conversion is done at the recording station 63, not in the control 1.